Shaking Conventional Wisdom; September 1993; Scientific American Magazine; by Kristin Leutwyler; 1 Page(s)

Those who adore Brazil nuts have no doubt wondered why shaking a can of assorted kernels always brings the large ones to the top. This somewhat counterintuitive ability of vigorous agitation to separate grains according to size, no matter how dense they are or what they are made of, has puzzled engineers and academics as well. Now a team of physicists from the University of Chicago reports it has discovered a mechanism entirely different from previous explanations.

Conventional wisdom holds that local avalanching causes the segregation by size: vibrations open gaps underneath the larger particles; smaller particles cascade into the voids, gradually pushing the biggest ones toward the surface. To test computer models of this idea, James B. Knight, H. M. Jaeger and Sidney R. Nagel decided to build their own "can of nuts": a cylinder 35 millimeters in diameter, filled with spherical glass beads two millimeters in diameter. The researchers added various numbers of larger beads, up to 25 millimeters in diameter, which were dyed so their movement could be traced. The container received a vertical shake, or "tap," once each second. "There was a wager as to whether the small beads rose with the larger beads as well," Knight says.